We are still no closer to establishing if this is a problem in Django or not. What do you have to do to replicate this problem. Merely posting the exception does not give steps to reproducing.

As Adrian mentioned, there is already a lot of code using that code path, so it must be something unusual that is being done to trigger this. A small example that demonstrates the problem would be ideal here.

Please notice that I am not the one who filed the bug. I just saw a production code hitting the same line that the patch changes and after going through the code I found out the reason was a call to a model constructor that used request.POST instead of form.cleaned_data